A conventional communication system has been known in which a short-range wireless communication device is installed in a vehicle, and settings of onboard equipment in the short-range wireless communication device are adjusted by use of an occupant's portable communication terminal. In this communication system, the short-range wireless communication device and the portable communication terminal need to be authenticated by short-range wireless communication. Specifically, the portable communication terminal is held close to an NFC (Near field Communication) antenna of the short-range wireless communication device, and the short-range wireless communication device and the portable communication terminal are authenticated by short-range wireless communication (see Japanese Patent Application Publication No. 2014-33534, for example).